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Community and Q&A

Moisture protection for sleepers over slab on grade

dfvellone | Posted in General Questions on

My location is zip code 13420.
My foundation is a monolithic slab on grade. I installed a 16″ gravel base with Vapor Block 15 above and 2″ xps on top of the Vapor Block. The perimeter of the slab is also insulated with 2″ xps. Before I closed the project in the slab would get soaked with condensation when outside temperatures and humidity rose to the extent that it looked like it was hosed down. Closing in greatly reduced that effect, but my roof isn’t yet closed off from the interior – no drop ceiling at 2nd floor yet and gables and rafter ends are wide open – and the space isn’t heated, so after prolonged high humidity the floor can eventually get damp. A portion of the slab will get sleepers and hardwood flooring and I’m wondering what the best approach would be for minimizing moisture against the bottom of the sleepers. Felt, or thin rolled foam, or even 1/2″ xps. My thoughts are that once the space is completely closed in, insulated and conditioned that I’ll get no condensation on the concrete, but I’m wondering, given the gravel, Vapor Block, and foam, do I need to be all that concerned about protecting the sleepers or will what little moisture may migrate into the concrete be negligible? Thanks, Daniel

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  1. JesseTrinque | | #1

    When I built, I did 10mil poly with 4" of foam on top sub slab. The slab was poured after the first floor deck went on and I experienced similar issues. Wet, wet concrete floor almost all the time during construction. Puddling on the floor in places. fast forward 8 months and we moved in. For the first two months i was horrified. The basement was crazy damp and grossly humid. I had put radiant tubing in the floor. I let the radiant run full time to try to cook the moisture out for a couple weeks. My only success there was burning through a couple hundred gallons of propane... Then I ran a dehumidifier in basement for a month on full blast, still couldn't control the humidity in the house and the basement was still nasty. Now we've been in for 4 months. Ive shut the dehumidifier off completely and the basement is dry as a bone. Seems the radiant tubing was a waste, the floor feels warm all the time. But the basement is the warmest driest part of the house. My advice.... Let it dry out. There is probably a TON of water in that slab and the vapor barrier/foam does not let it migrate down so until it bakes out it will be a problem but once it does you will be fine. That's been my experience anyway.

  2. GBA Editor
    Martin Holladay | | #2

    Jesse is right. You need patience -- patience to allow the slab to fully cure and dry out.

    You made a small blunder in your floor assembly. The Vapor Block 15 (your vapor barrier -- equivalent to polyethylene) should have been installed above the rigid foam layer, not below the rigid foam layer. It's possible that you trapped rain above the Vapor Block 15, in the gaps between the foam, before the slab was placed.

    Here's an article -- too late for you -- on the right way to do it: "Polyethylene Under Concrete Slabs."

    Don't waste time beating yourself up about the error. Eventually, the slab will dry out. But it may take a while.

    1. dfvellone | | #3

      Martin, one of the benefits of building the house myself is that I poured the slab 4 years ago. It's been under cover for 2 years now. My thought - if I'm correct - is that the bulk of moisture in the concrete has dried out, but I'm left wondering if a layer of protection still needs to be installed beneath the sleepers. Once the space is conditioned condensation won't be an issue, but given the layers - gravel, vapor barrier - would just a thin layer of felt offer sufficent protection? Thanks, Daniel

      1. JesseTrinque | | #4

        My slab was poured a year before the house was conditioned. I doubt much water has really evaporated out of your slab if it has not been conditioned regardless of its age. I made the same mistake as you by putting the Poly under the foam and since the bottom of my foam was a little below the top of my footing and the poly ran up over the footing and I (mistakenly) carefully taped all the poly seams, I think I effectively created a really nice sub-slab 'pond' with foam floating around in it. It did get rain water. It took 3-4 month of conditioning/dehumidification to dry it out. I bet once you do you that you can put the sleepers down without worry. I wouldn't cover that slab with floor until its dry though through a few months of conditioning. Until the slab dried out I couldn't keep the house below 65% RH. Now 4 moths later, having no issues maintaining around 30% with just normal ventilation. It was definitely all coming from the slab, moist to the touch.

        1. dfvellone | | #5

          Of all the aspects of my project I did my homework on the layers was one I left to my engineer! She had specified the gravel/vaporblock/ foam order. Anyways, I did meticulously tape the t+g foam when installing it.

  3. natesc | | #6

    I would consider 2 layers of 3/8 plywood laid perpendicular.

    Laying hardwood flooring is somewhat back breaking, I would not want to deal with sleepers + lots of additional splicing as you go. Hardwood flooring will generally come in random lengths of 1-8'.

    BTW since you are in Herkimer, you have local access to some of the best hardwood mills in the world. For flooring, both Wightman Lumber in Portlandville and Quality Hardwoods in Bainbridge have exceptional products.

    Wightman will also give wholesale pricing on lumber orders over 200BF. If you are willing and have the tools you could do things like up to 16' long cherry T&G plank floor for less cost than people pay for plywood flooring at the big box stores.

    1. dfvellone | | #7

      Thanks for the suggestions. I get the flooring from a local sawyer who makes it from my sawlogs, so lengths are based on my sleeper centers. I'd like to check out those businesses out though for future reference. Thanks.

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